1. Field of the Invention
This invention relates generally to valves having a housing defining a flow channel, an axially displaceable valve member and a pair of sealing or packing rings which cooperate with the valve member to control flow and prevent leakage. More particularly, the present invention relates to shut-off valves having first and second sealing rings and to sealing rings comprised of axially alternating layers of radially extending soft material lamellae and metal lamellae, the soft material lamellae having a non-homogeneous structure.
2. Description of the Prior Art
The soft material lamellae employed in prior art shut-off valves which employ a piston-type valve member are typically comprised of rubber-asbestos (IT) material or expanded graphite. Such soft materials are nonhomogeneous and consist of particles with interposed voids. Such voids prove to be a disadvantage when, in the operation of the valve, the operating system pressure suddenly drops. If such voids are filled with liquid, the sudden pressure drop may cause the liquid to flash to the vapor state. When this happens, the sudden increase in the volume of the contents of multiple voids results in explosive medium shock waves which propagate toward or away from the shell surface, i.e., the axial surface of the seal ring which contacts the valve member. The shock waves cause the soft material to flow, displacing parts of the seal ring sealing lamellae. Such relative displacements can result in sealing ring failure. The likelihood of this failure mode is particularly high in valves located upstream of condensate traps. In order to prevent seal ring damage, the addition of metal lamellae between layers of soft material lamellae to provide additional strength to the seal ring is well known in the art. However, such compound seal rings are still subject to damage from such explosive medium shocks, the shock waves causing the soft material lamellae to slide over the adjacent metal lamellae, thereby displacing parts of the sealing lamellae.